Radio frequency attenuator comprising a plurality of variable phase shifters and serially coupled directional couplers

ABSTRACT

A radio frequency attenuator including a ladder network made up of &#39;&#39;&#39;&#39;N&#39;&#39;&#39;&#39; serially coupled stages, each one of such stages having an output terminal coupled to a radio frequency source through a first and a second path, such second path having disposed therein a variable phase shifter. Included in each such stage is a load adapted to dissipate in the order of one Nth the power in the radio frequency energy. Each one of the phase shifters is responsive to a binary signal and the phase shift provided by each one of such phase shifters changes M 180* in accordance with the binary signal (where M is an odd integer).

United States Patent Fassett Dec. 2, 1975 RADIO FREQUENCY ATTENUATOR COMPRISING A PLURALITY OF VARIABLE PHASE SHIFTERS AND SERIALLY COUPLED DIRECTIONAL COUPLERS Inventor: Matthew Fassett, Billerica, Mass.

Assignee: Raytheon Company, Lexington,

Mass.

Filed: Aug. 21, 1974 Appl. No.: 499,390

US. Cl. 333/81 R; 333/l0; 333/81 B Int. Cl. H01? 1/22 Field of Search 333/10, 81 R, 81 A, 81 B;

References Cited UNITED STATES PATENTS 12/1966 Lowe 343/854 X 4/1970 Kinsey 343/854 COMPUTER 3,657,656 4/1972 Cooper 343/854 X Primary E.\'aminerPaul L. Gensler Attorney. Agent, or FirmRichard M. Sharkansky; Philip J. McFarland; Joseph D. Pannone [5 7] ABSTRACT A radio frequency attenuator including a ladder network made up of N serially coupled stages, each one of such stages having an output terminal coupled to a radio frequency source through a first and a second path, such second path having disposed therein a variable phase shifter. Included in each such stage is a load adapted to dissipate in the order of one Nth the power in the radio frequency energy. Each one of the phase shifters is responsive to a binary signal and the phase shift provided by each one of such phase shifters changes M 180 in accordance with the binary signal (where M is an odd integer).

2 Claims, 1 Drawing Figure US. Pltfiflt Dec. 2, 1975 mmCbnzzOu mumDOw u m RADIO FREQUENCY ATTENUATOR COMPRISING A PLURALITY OF VARIABLE PHASE SI-IIFTERS AND SERIALLY COUPLED DIRECTIONAL COUPLERS BACKGROUND OF THE INVENTION This invention relates generally to radio frequency attenuators and more particularly to high power variable radio frequency attenuators.

As is known in the art, radio frequency (RF) digitally controlled attenuators such as the type described in US. Pat. No. 3,765,020, inventors Seager et al., issued Oct. 9, 1973 and assigned to the same assignee as the present invention, include a ladder network wherein switching elements, each one responsive to digital signals, are selectively actuated to connect radio frequency energy desired to be attenuated to a load in such network. With means for equalizing the phase of radio frequency energy passing through the various paths possible in such a ladder network, it is possible then to control the attenuation of radio frequency energy by selectively actuating the switching elements. While attenuators of the just-mentioned type have been found to be adequate for many purposes, there are many applications where their use is not feasible at the present time. For example, if the radio frequency energy to be attenuated is at a relatively large power level, say in the order of several megawatts, each one of the switching elements should, to avoid the possibility of damage from overloading, be capable of switching radio frequency energy at a power level at least equal to the power level of the radio frequency energy to be attenuated. Therefore, while in theory it would be possible to provide switching elements having the requisite capacity to switch radio frequency energy at any given power level, the present state of the development of switching elements dictates that, if radio frequency energy at a high power level is to be attenuated, the use of an attenuator of the just-mentioned type is not practical.

SUMMARY OF THE INVENTION With this background of the invention in mind, it is an object of this invention to provide an improved variable attenuator for radio frequency energy having a power level up to several megawatts.

This and other objects of the invention are attained generally by providing a ladder network made up of N serially coupled stages for attenuating radio frequency energy, each one of such stages being adapted to dissipate in the order of one Nth of the power of such energy. To accomplish such end, each stage includes: A pair of input terminals; a pair of output terminals; a load; an adjustable phase shifter responsive to a control signal; and, a pair of directional couplers. The directional couplers each have one input coupled to a different one of the pair of input terminals, one output coupled to a different one of the pair of output terminals, and a second output, the second output of a first one of the directional couplers being coupled to the load and the second output of a second one of the directional couplers being coupled to a second input of the first one of such directional couplers through the adjustable phase shifter. In a preferred embodiment one of the output terminals of each of the stages is coupled to an RF source through a first path and a second path, such second path having disposed therein the variable phase shifter. Such phase shifter is responsive to a binary signal and the phase shift provided by such phase shifter changes M in accordance with the binary signal (where M is an odd integer).

BRIEF DESCRIPTION OF THE DRAWING Other objects and advantages of this invention will become apparent from the foregoing specification taken in connection with the accompanying single FIG- URE which shows a variable" radio frequency attenuator in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the FIGURE, a radio frequency source 10 (here producing a radio frequency signal of several megawatts power and a frequency of several megahertz) is shown coupled to a load 12 through an N stage digitally controlled radio frequency attenuator 14. Such radio frequency attenuator 14, the deiails of which will be described below, is adapted to attenuate the radio frequency energy supplied by the source 10 with a selected one of 2 attenuating factors ranging from 0.0 to 1.0, such factors being selected by a conventional digital computer 16.

Radio frequency attenuator 14 includes a 3 db coupler, here a conventional hybrid junction 16, having an H-arm 18 coupled to the radio frequency source, an E-arm 20 coupled to a suitable load 22, and a pair of side arms 24, 26. The side arms 24, 26 are connected, here by waveguide, to the first one of the N serially coupled stages 27 to 27 Each one of the first N-l stages 27 to 27 includes a pair of directional couplers: 30 32 30 32, as shown. The Nth stage 27 includes a single directional coupler 30 as shown. Each one of the directional couplers 30 to 30 and 32 to 32 is of conventional design and includes a pair of input ports i i and a pair of output ports 0 0 Directional couplers 30 to 30 N are serially coupled by the waveguide connecting the output port 0 of one of the couplers to the input port 1', of the adjacent one of such couplers. Likewise, directional couplers 32 to 32 .are serially coupled by waveguide connecting the output port 0 of one of the couplers to input port i of the adjacent one of such couplers. The output port 0 of the directional coupler 30- is coupled to the load 12. As is known, radio frequency energy introduced into input ports i and i is coupled to output ports 0 0 in accordance with the following relationship as described in detail in the article On the Design of Optimum Dual-Series Feed Networks by Jones et al., Transactions on Microwave Theory and Techniques, Volume MTT-l9, No. 5, May, 1971, pgs. 451, 453:

V, the voltage produced at the output port 0,

V the voltage produced at the output port 0 V the voltage introduced into input port V V the voltage introduced at input port V,

0 is the coupling index of the directional coupler and The output port 0 of each one of the directional couplers 30 to 30 is coupled to a different load (i.e. loads 38 to 38-) as shown. Such loads 38 to 38 are here designed, for reasons to be apparent, to handle l/Nth the power supplied by the radio frequency source 10. The

input ports i of directional couplers 32 to 32, are coupled to a different load, i.e. matching impedances 40 to 40- (as shown). Matching impedances 40 to 40- are designed to handle a lower amount of power than l/Nth the power supplied by the source because each simply provides conventional impedance matching.

Also included in the first N-1 stages are switchable phase shifters 42 to 42- such phase shifters being coupled between the input ports i of directional couplers 30 to 30 and the output ports 0 of directional couplers 32 to 32 as shown in the FIGURE. The Nth stage 27 includes a switchable phase shifter 42 coupled between the output port 0, of directional coupler 32 and the input port i of directional coupler 30 The switchable phase shifters 42 to 42- are coupled respectively, by way of control lines 44 to 44 to the digital computer 16. Such switchable phase shifters 42 to.42- are of any conventional design to change the radio frequency energy passing therethrough 180 in response to a change in the binary signal on the control line coupled thereto.

It is here noted that the radio frequency digitally controlled attenuator 14 is constructed so that: When the switchable phase shifter in any one of the stages, say stage 27m, is in state 0 (i.e. provides 0 phase shift to the radio frequency signals passing therethrough) the electrical length, via side arm 24, from l-l-arm 18 to the output port 0 of the directional coupler 30 of such stage differs from the electrical length, via side arm 26, from such l-l-arm to such output port 0 by MMZ (where M is an odd integer and A is the nominal operating wavelength of the radio frequency energy applied to the attenuator) and; when the switchable phase shifters are in state 1 (i.e. provides 180 phase shift to the radio frequency energy passing therethrough) the electrical lengths referred to above differ by MA. Such requirements may be achieved by proper waveguide length selection and/or by using conventional line stretchers (not shown). It follows then, assuming the directional couplers to be ideal lossless devices, that by proper selection of the coupling index, 6, for each one of the directional couplers: (a) When all of the switchable phase shifters are in the state 0 condition, the radio frequency energy applied to the hybrid junction 16 may be distributed equally between the loads 38 to 38 (b) When all of the switchable phase shifters are all in the 1 state, all of the radio frequency energy from the source is transferred to the load 12; and (0) Finally,

each different one of the 2 combination of states of the switchable phase shifters causes the radio frequency energy to be divided between the load 12 and some of the loads 38, to 38 In other words the state of the individual ones of the switchable phase shifters may be changed to vary the attenuation of radio frequency energy passed through the illustrated attenuator.

In order to more completely understand the attenuator, and more particularly the derivation of the coupling indices of the directional couplers, let us consider the following: When all of the stages are in the state 1 condition, the relationship between input voltages and output voltages of any one of the stages, say stage 27m, may be represented as follows:

V,- voltage on terminal i, of directional coupler 30,,

V voltage on terminal i of directional coupler 32, V voltage on terminal 0 of directional coupler 30, V voltage on terminal 0 of directional coupler 30 V voltage on terminal 0 of directional coupler 32 and, 0 coupling index for directional coupler 30 6 coupling index for directional coupler 32 (superscript (1) indicates state 1) When all of the stages are in the state 0 condition, the following relationships may be represented:

(superscript (0) indicates state 0) Because when all of the stages are in the state 1 condition V 0, then (from Eq. 4)

V, sin 0 V sin 0 cos 6, (9) or sin 0 V,;"/ v. tan 0, (10) Because when all of the stages are in the state 0 condition V K V (where: V is the voltage of the signal applied to the attenuator; K 1 IN; and, N the number of stages), then VTZK2 V sin 0, +(2V,1) V,2 sin 0, cos 0, sin

0 V sin 0 cos 0, Therefore,

VT2K2 lsinz 0 lfl) V1(0)]2 OI i -1 KVTK um la (13) Let us consider for purposes of example of 10 stage attenuator assuming it to have lossless phase shifters and couplers.

Therefore, from the above equation, for stage 27 WK .1v and, 14 v,,w= v 1 /2 For succeeding stages:

It should be noted that, as is known, the above coupling indices would be adjusted in accordance with the losses in the directional couplers and phase shifters.

While the described embodiments of the invention are useful to an understanding thereof, it will be immediately apparent to those having ordinary skill in the art that other embodiments are also covered by the inventive concepts disclosed herein. It is felt, therefore, that the invention should not be restricted to its disclosed embodiments but rather should be limited only by the spirit and scope of the following claims.

What is claimed is:

1. A radio frequency attentuator for coupling a source of radio frequency energy to an output load with an amount of attenuation selected in accordance with signals provided by a control signal source, comprising: a. a first plurality of serially coupled directional couplers, the last one thereof being adapted for coupling to the output load; b. a plurality of loads, each one thereof being coupled to a different one of the directional couplers;

c. a plurality of variable phase shifters, each one thereof being coupled to a different one of the directional couplers and to the control signal source;

d. a second plurality of directional couplers, each one thereof being coupled to a different one of the first plurality of directional couplers through a different one of the variable phase shifters; and

e. a power divider having an input terminal adapted for coupling to the source of radio frequency energy, and a pair of output terminals, one of such output tenninals being coupled to the first one of the first plurality of serially coupled directional couplers and the other one of the pair of output terminals being coupled to the first one of the second plurality of serially coupled directional couplers.

2. The radio frequency attenuator recited in claim 1 wherein each one of the directional couplers in the first plurality thereof is coupled to the pair of output terminals of the power divider through two paths, one of such two paths having one of the phase shifters disposed therein, and wherein the control signal source is adapted to set the one of the phase shifters in a selected one of two phase states, one of such phase states providing M( (where M is an odd integer) phase shift to radio frequency energy passing therethrough relative to the phase shift provided to such energy when such phase shifter is set in the other one of such phase states. 

1. A radio frequency attentuator for coupling a source of radio frequency energy to an output load with an amount of attenuation selected in accordance with signals provided by a control signal source, comprising: a. a first plurality of serially coupled directional couplers, the last one thereof being adapted for coupling to the output load; b. a plurality of loads, each one thereof being coupled to a different one of the directional couplers; c. a plurality of variable phase shifters, each one thereof being coupled to a different one of the directional couplers and to the control signal source; d. a second plurality of directional couplers, each one thereof being coupled to a different one of the first plurality of directional couplers through a different one of the variable phase shifters; and e. a power divider having an input terminal adapted for coupling to the source of radio frequency energy, and a pair of output terminals, one of such output terminals being coupled to the first one of the first plurality of serially coupled directional couplers and the other one of the pair of output terminals being coupled to the first one of the second plurality of serially coupled directional couplers.
 2. The radio frequency attenuator recited in claim 1 wherein each one of the directional couplers in the first plurality thereof is coupled to the pair of output terminals of the power divider through two paths, one of such two paths having one of the phase shifters disposed therein, and wherein the control signal source is adapted to set the one of the phase shifters in a selected one of two phase states, one of such phase states providing M(180*) (where M is an odd integer) phase shift to radio frequency energy passing therethrough relative to the phase shift provided to such energy when such phase shifter is set in the other one of such phase states. 